The invention relates to a cellular radio network and a method for dividing traffic into different cells in the cellular radio network.
One of the key problems when constructing and maintaining cellular radio networks is the limited scope of the available radio spectrum. The aim is to minimize interference caused by a co-channel signal and an adjacent channel signal by carefully planning the use of radio frequencies. The frequencies are divided according to various complex models into different cells with the intention to minimize the interference occurring on the radio connections, thus maximizing the network capacity. In the same or adjacent repetition pattern of a cell, the frequencies cannot be too close to each other, because this causes excessive interference to the system. As the use of mobile phones and other subscriber terminals becomes more common, the capacity of networks must continuously be increased. This incurs high costs in frequency planning and various measurements.
A strict repetition pattern of frequencies causes in current networks, such as the GSM network, the network capacity to be interference limited. Consequently the interference level of the network, and therefore the number of disconnected connections, increases significantly, when the network load exceeds a particular threshold. The capacity of an interference limited network can be maximized if the interference can be equally divided into different cells of the network. In an optimal situation, no good, intermediate or poor quality signals exist, but all connections are xe2x80x9cgood enoughxe2x80x9d.
A known method for dividing traffic as well as interference in a network is handovers, which are performed on account of the traffic (TrHO, Traffic reason Handover). In such a case, a number of terminals is controlled to perform handover from a loaded cell to an adjacent less loaded cell. This is illustrated in FIG. 1. FIG. 1 shows a set of cellular radio system cells 100 to 106. The intermediate cell 100 is heavily loaded but the surrounding cells 102 to 106 include less traffic. Then some of the terminals 108 to 114 of the cell can be directed to perform handover to a stronger neighbouring cell including capacity.
Another known method to balance interference in the system is a dynamic Hot Spot method (DHS, Dynamic Hot Spot). This is illustrated in FIG. 2. FIG. 2 shows a set of cells 200 to 206 in the cellular radio system. Always when a need arises in the network to perform either handover or allocation of a new call, the frequencies used in various cells are checked, and how the cell, into which handover is made or in which a new call is formed, interferes with the surrounding cells using the same frequency. In the example shown in FIG. 2, a new call 208 is to be established in the cell 200 to a terminal 210. It is observed that the cells 202 and 204 use the same frequencies. In this case the interference caused by the cell 200 is checked in the cells 202 and 204. If the cell 200 causes a significant amount of interference to said cells and the connection quality in these cells 202 and 204 is poor, permission for a new connection is not granted. The basic idea is thus that when the load of a particular cell exceeds a predetermined value and the interference in the rest of the cellular radio network therefore increases, then so-called soft blocking can be used in channel allocation, i.e. the channel is not implemented due to the increased amount of interference even if it were available. The blocking criterion is checked in each new call allocation and inter-cell handover.
Prior art methods have, however, several drawbacks. The handovers performed on the basis of traffic tend to move traffic from a loaded cell into less loaded cells. Then channels are released from the loaded cells. In an interference limited network, on account of the strict frequency pattern, the problem is not the number of free channels but the cell quality, which varies significantly, and is caused by the amount of interference in the co-channel sent by an interfering cell (interfering cells) and the location of the mobile stations of said cells (the radio path between the base station and the mobile station). For example, if the connection of the terminal includes a strong specific signal, an interfering signal may also be strong, whereas the connection including a weak specific signal, cannot allow a lot of interference. In an interference limited network the interference should be divided in such a manner that the cells, in which the connection quality is good can be allowed more co-channel interference, and the cells, in which the connection quality is poor should reduce co-channel interference. Hence, interference is not necessarily balanced (to correspond in numbers) in each cell but divided in accordance with the situation (based on measurements) between the cells. However, the TrHO method does not take into account the interference caused by the target cell to the surrounding cells when performing handover. If the interfered cells on every connection are of good quality, handovers need not be performed from an interfering cell having a heavy traffic load. Because, even if the target cell had a smaller traffic load, the interference thereof may cause poorer connection qualities to the surrounding co-channel cells. The TrHO method is therefore not very efficient.
In the DHS method the number of new calls and handovers to be accepted in the cell is restricted on the basis of the interference caused by the cell. When adjacent cells overlap considerably, the rejection of handovers does not necessarily become a problem, but if terminals move rapidly and the coverage areas of the adjacent cells do not overlap considerably, then the rejection of handover may result in a disconnected call. This occurs in particular if a terminal moves towards a cell that interferes with other cells. Then, although the terminal connection to the base station thereof fades, it cannot perform handover to a new cell interfering with the others, and then the call is disconnected. Therefore, the DHS method rather restricts the creation of interferences than balances the interferences to different cells.
It is an object of the invention to provide a method allowing to divide interference to different cells avoiding the above problems. This is achieved by the method of the invention for dividing traffic between cells in a cellular radio network, where a weighting value based on network measurements is calculated for each cell. In the method of the invention the size of each cell is adjusted on the basis of the weighting value calculated for the cell.
The invention also relates to a cellular radio system comprising at least a base station of a particular cell, at least one subscriber terminal communicating with the base station, and cells surrounding the particular cell, and subscriber terminals communicating therewith, and the cellular radio system being arranged to measure the amount of traffic and interference in the system, and to calculate a weighting value for each cell on the basis of the network measurements. In the system of the invention the base station of the particular cell is arranged to adjust the size of its coverage area on the basis of the weighting value of the cell.
The preferred embodiments of the invention are disclosed in the dependent claims.
Several advantages are achieved with the solution of the invention. In a preferred embodiment of the invention the size of the cell is adjusted by dynamically changing the handover margin of the cell in relation to the surrounding cells. On this account the terminals in the border area between the cells can preferably be directed to the best cell as regards interferences. The invention allows to avoid calls to be disconnected, since handovers are not refused. By adjusting the handover margin the moment for performing handover can be selected so as to minimize interference.
In another preferred embodiment of the invention the handover margin of the cell has a specific base value, and the margin is changed using a correction factor depending on the amount of interference.
In a further embodiment of the invention the solution of the invention is applied together with the DHS method. Then the connections subjected to interferences from the cell are determined on the basis of the measurements of the subscriber terminals located in the neighbouring cells concerning the strongest neighbouring cells, and the frequencies used by the neighbouring cells and common to the cell to be examined. The correction factor of the handover margin depends on the connection quality of each surrounding cell subjected to interference. Then the actual interference can be very accurately estimated and the interference can be balanced between different cells more accurately than before.